Method and apparatus for controlling transmission power in WLAN system

ABSTRACT

A transmit power control method of a first station in a wireless LAN (WLAN) system includes: receiving path loss information from an (access point) AP, the path loss information containing a maximum value among path losses between the AP and at least one or more stations included in the WLAN system; controlling a transmit power by using the path loss information; and transmitting a frame according to the controlled transmit power. In said controlling the transmit power by using the path loss information, the transmit power is controlled by using a path loss obtained by adding the maximum value and a path loss between the first station and the AP or by using the path loss between the first station and the AP.

CROSS-REFERENCE(S) TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 12/910,718, filed on Oct. 22, 2010, which claims priority ofKorean Patent Application No. 10-2009-0101232, filed on Oct. 23, 2009,which are incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Exemplary embodiments of the present invention relate to a transmitpower control method and apparatus in a wireless LAN (WLAN) system; and,more particularly, to a transmit power control method and apparatuswhich controls a transmit power by using a path loss between a stationand an access point (AP) in a WLAN system.

2. Description of Related Art

In a wireless communication device, a transmission terminal requires2-2.5 times larger power consumption than a reception terminal, and thepower consumption required by the transmission terminal dependssignificantly on the magnitude of transmit power. In general, acommercial wireless LAN device focuses on the transmission performancerather than the power consumption. Therefore, when transmitting a frame,the commercial wireless LAN device sets a transmit power to the maximumtransmit power.

When a portable device uses the maximum transmit power, the powerconsumption efficiency of a wireless station may become a problem. Adynamic transmit power control function of the wireless communicationsystem for the power consumption efficiency may not only reduce batteryconsumption, but also improve the immunity to interference.

Conventionally, research has been conducted on a transmit power controlmethod between a station and an AP or between a station and a station.However, an algorithm combined with transmit rate control is required interms of link control, and a transmit power control scheme is requiredin which a hidden node problem and a basic service set (BSS) networkunit are simultaneously considered.

The hidden node problem occurs when a signal transmitted from a specificstation within a basic service set suffers from a path loss caused by achannel and does not approach the receive sensitivity of anotherstation. In this case, when a station occupies a channel, anotherstation may not recognize the occupation. Furthermore, as anotherstation attempts to transmit a signal, signal collision may occur. Inorder to minimize the hidden node problem, ready-to-send (RTS) andclear-to-send (CTS) frames are used. After the RTS and CTS frames aretransmitted at the maximum power, a station may occupy a channel andacquire a transmit opportunity. In this case, the station may transmitdata at a proper transmit power during the acquired transmitopportunity.

During frequent data communication processes, the exchange of RTS andCTS frames using the maximum transmit power may serve as a main factorwhich reduces the throughput efficiency and power consumption efficiencyof a wireless station. Furthermore, the performance of a stationincluded in the radio range of an AP belonging to different BSSs may bereduced, and the performance of another station may be reduced by astation belonging to an overlapping basic service set (OBBS). In a caseof the next-generation WLAN system, the bandwidth may be widened, and amulti-mode and multi-user scheme may be used. Therefore, it is expectedthat the interference problem will be serious in the next-generationWLAN system.

SUMMARY OF THE INVENTION

An embodiment of the present invention is directed to a transmit powercontrol method and apparatus in a WLAN system, which is capable ofimproving power consumption efficiency while maintaining communicationperformance.

Another embodiment of the present invention is directed to a transmitpower control method and apparatus in a WLAN system, which is capable ofsolving a hidden node problem and a problem caused by an OBSS.

Other objects and advantages of the present invention can be understoodby the following description, and become apparent with reference to theembodiments of the present invention. Also, it is obvious to thoseskilled in the art to which the present invention pertains that theobjects and advantages of the present invention can be realized by themeans as claimed and combinations thereof.

In accordance with an embodiment of the present invention, a transmitpower control method of a first station in a WLAN system includes:receiving path loss information from an AP, the path loss informationcontaining a maximum value among path losses between the AP and at leastone or more stations included in the WLAN system; controlling a transmitpower by using the path loss information; and transmitting a frameaccording to the controlled transmit power. In said controlling thetransmit power by using the path loss information, the transmit power iscontrolled by using a path loss obtained by adding the maximum value anda path loss between the first station and the AP or by using the pathloss between the first station and the AP.

In accordance with another embodiment of the present invention, atransmit power control method of a first station in a WLAN systemincludes: acquiring a first path loss between the first station and atleast one or more stations included in the WLAN system and a second pathloss between the first station and an AP; controlling a transmit powerby using the first and second path losses; and transmitting a frameaccording to the controlled transmit power. In said controlling thetransmit power by using the first and second path losses, the transmitpower is controlled by using a maximum value of the first and secondpath losses or by using the second path loss.

In accordance with another embodiment of the present invention, atransmit power control method of an AP in a WLAN system including atleast one or more stations includes: acquiring a path loss between theAP and the station; controlling a transmit power according to a maximumvalue among the path losses; and transmitting a frame according to thecontrolled transmit power.

In accordance with another embodiment of the present invention, atransmit power control apparatus includes: a reception unit configuredto receive path loss information from an AP, the path loss informationcontaining a maximum value among path losses between the AP and at leastone or more stations included in a WLAN system; and a control unitconfigured to control a transmit power by using the path lossinformation. The control unit controls the transmit power by using apath loss obtained by adding the maximum value and a path loss betweenthe first station and the AP or by using the path loss between the firststation and the AP.

In accordance with another embodiment of the present invention, atransmit power control apparatus includes: an acquisition unitconfigured to acquire a first path loss between a station including thetransmit power control apparatus and at least one or more stationsincluded in a WLAN system and a second path loss between the stationincluding the transmit power control apparatus and an AP; and a controlunit configured to control a transmit power by using the first andsecond path losses. The control unit controls the transmit power byusing a maximum value of the first and second path losses or by usingthe second path loss between the station including the transmit powercontrol apparatus and the AP.

In accordance with another embodiment of the present invention, atransmit power control apparatus in a WLAN system including at least oneor more stations includes: an acquisition unit configured to acquire apath loss between the station and an AP including the transmit powercontrol apparatus; and a control unit configured to control a transmitpower according to a maximum value among the path losses.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram explaining the concept of a transmit power controlmethod in accordance with an embodiment of the present invention.

FIGS. 2A and 2B are diagrams illustrating radio ranges in accordancewith a conventional method and the radio ranges of a high-speed mode anda low-power mode in accordance with the embodiment of the presentinvention.

FIG. 3 is a flow chart showing a transmit power control method of afirst station in accordance with an embodiment of the present invention.

FIG. 4 is a flow chart showing a transmit power control method of afirst station in accordance with another embodiment of the presentinvention.

FIG. 5 is a flow chart showing a transmit power control method of an APin accordance with another embodiment of the present invention.

FIG. 6 is a diagram explaining a process of solving a hidden nodeproblem in accordance with the embodiment of the present invention.

DESCRIPTION OF SPECIFIC EMBODIMENTS

Exemplary embodiments of the present invention will be described belowin more detail with reference to the accompanying drawings. The presentinvention may, however, be embodied in different forms and should not beconstructed as limited to the embodiments set forth herein. Rather,these embodiments are provided so that this disclosure will be thoroughand complete, and will fully convey the scope of the present inventionto those skilled in the art. Throughout the disclosure, like referencenumerals refer to like parts throughout the various figures andembodiments of the present invention.

A transmit power control method and apparatus in accordance with anembodiment of the present invention controls a transmit power by using apath loss between an AP and a station. As the path loss is used, a framedoes not need to be transmitted at the maximum power, but the transmitpower may be controlled to transmit a frame.

For example, when a first station knows a path loss between the firststation and a second station which is the most remote from the firststation, the first station does not need to transmit a frame at themaximum transmit power, but may control the transmit power according tothe path loss and transmit a frame. That is, the first station maycontrol the transmit power such that the radio range of the firststation does not exceed the second station. In this case, thecommunication performance may be maintained, and the power consumptiondepending on the frame transmission may be reduced.

That is, the transmit power control method and apparatus in accordancewith the embodiment of the preset invention may improve the powerconsumption efficiency by controlling the transmit power according tothe path loss.

Meanwhile, the transmit power control method and apparatus in accordancewith the embodiment of the present invention may control the transmitpower while operating in a high-speed mode or a low-power mode. In thehigh-speed mode, data frames are transmitted and received without usingRTS and CTS frames. Since RTS and CTS frames are not used in thehigh-speed mode, data may be transmitted at a high speed. In thelow-power mode, RTS and CTS frames are used to transmit and receive dataframes. That is, the transmit power control method and apparatus inaccordance with the embodiment of the present invention may control thetransmit power by using the high-speed mode or the low-power modedepending on the frames.

Meanwhile, a mode in which frames are transmitted and received at themaximum transmit power is referred to as a maximum power mode. Thetransmit power control method depending on the high-speed mode or thelow-power mode may differ according to a path loss to be used.

Hereafter, the transmit power control method and apparatus in accordancewith the embodiment of the present invention will be described in moredetail with reference to the drawings.

FIG. 1 is a diagram explaining the concept of a transmit power controlmethod in accordance with an embodiment of the present invention.

FIG. 1 shows a transmit power control method in a WLAN system includingone AP 101 and two stations 103 and 105. Furthermore, FIG. 1 shows twoexamples of the transmit power control method depending on path lossesto be used. First, the first example will be described, and the secondexample will be then described.

In accordance with the first example, path losses between the AP 101 andthe stations 103 and 105 are used. That is, the path loss between the AP101 and the station 103 and the path loss between the AP 101 and thestation 105 are used. The path loss may be measured by the station orthe AP.

The AP 101 transmits a maximum value of the path losses to the first andsecond stations 103 and 105. The first station 103 may control thetransmit power by using a path loss obtained by adding the maximum valueand the path loss between the first station 103 and the AP 101 or byusing the path loss between the first station 103 and the AP 101,depending on the high-speed mode or the low-power mode.

Since RTS and CTS frames are not used in the high-speed mode, the firststation 103 controls the transmit power, based on the path loss obtainedby adding the maximum value and the path loss between the first station103 and the AP 101. In FIG. 1, the maximum path loss is a path lossbetween the AP 101 and the second station 105. That is, the firststation 103 controls the transmit power, based on a path loss obtainedby adding the path loss between the AP 101 and the first station 103 andthe path loss between the AP 101 and the second station 105. The pathloss increases in proportional to a signal transmission distance.

The first station 103 may transmit a data frame to the AP 101 accordingto the transmit power controlled by the above-described method. That is,the first station 103 may control the transmit power, based on a pathloss corresponding to a distance (d1+d2), and transmit a data frame tothe AP 101 according to the controlled transmit power.

A distance d3 between the first station 103 and the second station 105is smaller than the distance (d1+d2). Therefore, in accordance with thefirst example, the second station 105 may be included in thetransmission signal range of the first station 103. Accordingly,although the RTS and CTS frames are not used, the second station 105 maydetect a signal of the first station 103. Therefore, a signal collisionproblem caused by a hidden node problem may be solved, and the transmitpower may be reduced. Furthermore, since the RTS and CTS frames are notused, the throughput may be improved.

Next, when the first station 103 transmits an RTS frame in the low-powermode, the first station 103 may control the transmit power, based on apath loss obtained by adding the maximum value and the path loss betweenthe first station 103 and the AP 101. Alternatively, the first station103 may transmit an RTS frame at the maximum transmit power. Then, whenthe first station 103 acquiring a transmission opportunity transmits adata frame, the first station 103 controls the transmit power, based onthe path loss between the first station 103 and the AP 101.

In the low-power mode, the first station 103 acquires a transmissionopportunity by using the RTS frame. Therefore, when transmitting a dataframe, the first station 103 does not need to control the transmitpower, based on the path loss obtained by adding the maximum value andthe path loss between the first station 103 and the AP 101. That is,when transmitting a data frame in the low-power mode, the first station103 does not need to control the transmit power as in the high-speedmode.

Meanwhile, the AP 101 may transmit the maximum value and the secondlargest value among the path losses to the first and second stations 103and 105. The second largest value is transmitted for the sake of thesecond station 105 corresponding to the maximum path loss. The secondstation 105 may control the transmit power, based on a path lossobtained by adding the maximum value and the second largest value amongthe path losses. The second station 105 may control the transmit power,based on a path loss corresponding to double the maximum value among thepath losses. In this case, however, a loss may occur in the transmitpower, compared with the former case.

In accordance with the second example, path losses among all WLANdevices included in the WLAN system are used. That is, the path lossbetween the AP 101 and the first station 103, the path loss between theAP 101 and the second station 105, and the path loss between the firststation 103 and the second station 105 are used.

In the second example, different path losses are used in comparison withthe first example, but the basic transmit power control method issimilar to that of the first example. However, the first example has anadvantage in that the design is simple and data may be transmitted at ahigh speed because the RTS and CTS frames are not used. The secondexample has an advantage in that the transmit power may be minimized.

In the case of the high-speed mode or the RTS frame transmission in thefirst example, the first station 103 transmits a frame according to thetransmit power corresponding to the distance (d1+d2). In the secondexample, however, the first station 103 may transmit a frame accordingto a transmit power corresponding to a distance d3. That is because thepath loss between the first station 103 and the second station 105 isused in the second example. Therefore, in accordance with the secondexample, it is possible to minimize the transmit power, compared withthe first example.

Hereafter, the second example will be described in detail.

The first station 103 acquires a first path loss between the firststation 103 and the second station 105 and a second path loss betweenthe first station 103 and the AP 101. Then, the first station 103controls a transmit power by using the first and second path losses.Specifically, the first station 103 may control the transmit power byusing a maximum value of the first and second path losses or control thetransmit power by using the second path loss, depending on thehigh-speed mode or the low-power mode.

In the high-speed mode, RTS and CTS frames are not used. Therefore, thefirst station 103 controls the transmit power, based on the maximumvalue of the first and second path losses. The first station 103 maytransmit a data frame to the AP 101 according to the transmit powercontrolled by the above-described method. In FIG. 1, the maximum valueis the first path loss. That is, the first station 103 may control thetransmit power, based on the path loss corresponding to the distance d3,and transmit a data frame to the AP 101 according to the controlledtransmit power.

In accordance with the second example, the second station 105 isincluded in the transmission signal range of the first station 103.Therefore, although RTS and CTS frames are not used, the second station105 may detect a signal of the first station 103. Accordingly, a signalcollision problem caused by a hidden node problem may be solved, and thetransmit power may be reduced. Furthermore, since RTS and CTS frames arenot used, the throughput may be improved.

Next, when transmitting an RTS frame in the low-power mode, the firststation 103 may control a transmit power, based on the maximum value ofthe first and second path losses. Alternatively, the first station 103may transmit an RTS frame at the maximum transmit power. Then, when thefirst station 103 acquiring a transmission opportunity transmits a dataframe, the first station 103 controls the transmit power, based on thesecond path loss.

So far, the transmit power control method of the first station 103 hasbeen described. However, the second station 105 may also control atransmit power, like the first station 103.

Meanwhile, the AP 101 controls a transmit power according to a maximumvalue of the path losses between the AP 101 and the first and secondpath losses 103 and 105. That is, the AP 101 may acquire the path lossbetween the AP 101 and the first station 103 and the path loss betweenthe AP 101 and the second station 105, and control the transmit poweraccording to the maximum value of the acquired path losses.

In the WLAN system, communication is centered around the AP 101.Therefore, as the AP 101 controls the transmit power based on themaximum path loss, the power consumption may be reduced, and frames maybe transmitted and received without signal collision. In FIG. 1, themaximum path loss based on the AP 101 is the path loss between the AP101 and the second station 105. That is, the AP 101 controls thetransmit power according to the path loss corresponding to the distanced2, and transmits frames to the first and second stations 103 and 105according to the controlled transmit power. The first station 103 may beincluded in the transmission signal range of the AP 101.

The above-described high-speed and low-power modes may be selectedaccording to the following standards, for example. The high-speed andlow-power modes may be selected depending on whether the transmit rateis preceded or the power consumption is preceded. When the transmit rateis preceded, the high-speed mode may be selected in case where trafficis larger than a preset threshold value, and the low-power mode may beselected in case where traffic is smaller than the preset thresholdvalue. When the power consumption is preceded, the high-speed mode maybe selected in case where the power amount of a station is larger than apreset threshold value, and the low-power mode may be selected in casewhere the power amount is smaller than the preset threshold value.

FIGS. 2A and 2B are diagrams illustrating radio ranges in accordancewith a conventional method and the radio ranges of the high-speed modeand the low-power mode in accordance with the embodiment of the presentinvention.

FIG. 2A illustrates the radio ranges of an access point and second andthird stations in the conventional method. The radio ranges of FIG. 2Adepend on the maximum power mode. FIG. 2B illustrates the radio rangesof second and third stations in the high-speed mode and the low-powermode and the radio range of an AP in accordance with the embodiment ofthe present invention. In FIG. 2B, the radio range in the low-power modeindicates a radio range in which data frames and response frames aretransmitted and received, and the radio range of the low-power mode isnarrower than that of the high-speed mode.

In FIGS. 2A and 2B, the distances between the AP and the respectivestations are equal to each other. That is, it can be seen in FIGS. 2Aand 2B that the radio ranges of the high-speed mode and the low-powermode are narrower than the radio range according to the maximum powermode, and all the stations are included in the radio ranges. Inaccordance with the embodiment of the present invention, the signalcollision problem may be solved, and the transmit power may be reduced,compared with the conventional method.

Meanwhile, the transmit power control method according to the path lossmay be expressed as equations below. Hereafter, the transmit powercontrol method between the first and second stations will be taken as anexample.PL=TPG1−RSSI1  Eq. 1

For example, when the first station transmits a first frame to thesecond station, the path loss between the first and second stations maybe expressed as Equation 1 above. In Equation 1, PL represents the pathloss between the first and second stations, TPG1 represents a transmitpower gain (TPG) included in the first frame transmitted by the firststation, and RSSI1 represents a received signal strength indicator(RSSI) measured by the second station having received the first frame.That is, the second station may calculate the path loss by using the TPGand the RSSI included in the first frame transmitted from the firststation.TPG2=RSSI2+PL+RG  Eq. 2

In Equation 2, TGP2 represents a TPG for a second frame transmitted tothe first station, after the second station receives the first frame,and RSSI2 represents an RSSI included in the first frame received fromthe first station by the second station. When the channel states orstation characteristics are identical, RSSI1 and RSSI2 may be equal toeach other. PL represents the path loss calculated by Equation 1, and RGis a gain for satisfying required performance. That is, the secondstation may control the transmit power as in Equation 2 by using thepath loss calculated by Equation 1, and transmit a frame at the transmitpower such as TGP2.

As described above, a TPG and a RSSI may be included in transmitted andreceived frames, in order to acquire a path loss. Alternatively, areceiver receiving a frame may acquire a path loss by measuring an RSSI.Alternatively, a transmitted frame including transmission timeinformation may be used to calculate a transmission/reception timedifference as in the conventional method, thereby acquiring a path loss.

In the first example of FIG. 1, the first station 103 may generate thepath loss between the first station 103 and the AP 101 by using a TPGincluded in a frame transmitted from the AP 101 and an RSSI for theframe transmitted from the AP 101. Furthermore, the first station 103may transmit the path loss between the first station 103 and the AP 101to the AP 101. The AP 101 may retransmit the maximum value, among thepath losses transmitted from the stations, to the stations.

In the second example of FIG. 1, the first station 103 may acquire firstand second path losses by using TPGs included in frames transmitted fromat least one or more stations 105 or the AP 101 included in the WLANsystem and RSSIs for the frames transmitted from at least one or morestations 105 or the AP 101 included in the WLAN system.

Meanwhile, the transmit power control method in accordance with theembodiment of the present invention may control the transmit power byadditionally considering a variable transmit rate. In a general WLANsystem, the transmit rate is not fixed, but a link adaption method isused. In the link adaption method, the transmit rate is controlleddepending on a channel state variation or the like. The transmit powercontrol method in accordance with the embodiment of the presentinvention additionally controls the transmit power depending on thetransmit rate, that is, a data rate. This may be expressed as Equation 3below.TPG3=RSSI2+PL+RG+AG  Eq. 3

In Equation 3, TPG3 represents a TPG for a third frame which the secondstation transmits after transmitting the second frame according toEquation 2, and AG represents a gain for increasing or reducing thetransmit rate. That is, the second station may control the transmitpower by using the AG as well as the path loss.

In order to increase the transmit rate while maintaining the radioranges at the same level, the transmit power needs to be increased.Therefore, the second station may increase the transmit rate byincreasing the AG. Furthermore, when the data rate of the second stationis reduced, the transmit power of the second station may be furtherreduced while the radio range of the second station is maintained.

Meanwhile, a modulation and coding set (MCS) level depending on thetransmit rate may be determined as expressed by Equation 4 below. InEquation 4, RC represents a transmit rate control value based on the AG.Furthermore, an MCS level indicates a value which increases as thetransmit rate increases. Current MCS level represents an MCS leveldepending on a current data transmit rate, and Next MCS level representsan MCS level depending on a next data transmit rate.Next MCS=Current MCS+RC  Eq. 4

The second station may determine a proper MCS level according to apredetermined TPG. The RSSI of a reception terminal for a signal to betransmitted may be determined by the sum of the predetermined TPG of thesecond station and the AG. Therefore, the MCS level may be determined byestimating the RSSI according to the magnitude of the AG.

Meanwhile, according to the IEEE 802.11n standard, the first stationtransmits a response frame including its MCS information. The secondstation may select a minimum value between the MCS information includedin the response frame and the MCS level of the second station, set theminimum value to an MCS value according to Equation 5 below, and thentransmit a data frame. The MCS information included in the responseframe is referred to as a first MCS value, and the MCS level of thesecond station is referred to as a second MCS value.Determined MCS=Min(first MCS value, second MCS value)  Eq. 5

In Equation 5, Min (A, B) indicates a minimum value between A and B.

That is, the second station receives the data rate information from thefirst station having received the frame transmitted from the secondstation. Furthermore, the second station may transmit a frame accordingto a smaller data rate between the data rate of the second station andthe data rate of the first station, based on the predetermined transmitpower. The first station transmitting a response frame may also set anMCS level like the second station.

The smaller the MCS level, the higher the reliability for channelvariation. Therefore, in accordance with the embodiment of the presentinvention, the reliability for channel variation may be improved. Thatis, the open loop scheme in accordance with the embodiment of thepresent invention is combined with a close-loop-based fast link adaptionscheme using a response frame which has been presented in the standards.Furthermore, as the smaller value of two MCS levels is used, thecommunication may be performed in a state which is more stably adaptedfor channel variation.

Hereafter, the transmit power control methods of the station and the APwill be described in more detail on the basis of the transmit powercontrol method described with reference to FIGS. 1 and 2.

FIG. 3 is a flow chart showing a transmit power control method of afirst station in accordance with an embodiment of the present invention.

Referring to FIG. 3, the transmit power control method in accordancewith the embodiment of the present invention starts from step S301. Atthe step S301, the first station receives path loss information from anAP. The path loss information includes a maximum value among path lossesbetween the AP and at least one or more stations included in the WLANsystem. The first station may transmit a path loss between the firststation and the AP to the AP, and the stations included in the WLANsystem may also transmit the path losses between the stations and the APto the AP. Therefore, the AP may transmit the maximum value, among thepath losses received from the stations, to the first station.

Meanwhile, the first station may generate the path loss between thefirst station and the AP by using a TPG included in the frametransmitted from the AP and an RSSI for the frame transmitted from theAP.

At step S303, the first station controls a transmit power by using thepath loss information. At this time, the first station controls thetransmit power by using a path loss obtained by adding the maximum valueand the path loss between the first station and the AP or by using thepath loss between the first station and the AP.

At step S305, the first station transmits a frame according to thecontrolled transmit power.

More specifically, when the first station operates in the high-speedmode, that is, a transmitted frame is a data frame or response frame atthe step S303, the first station may control the transmit power, basedon the path loss obtained by adding the maximum value and the path lossbetween the first station and the AP. Furthermore, when a transmittedframe is an RTS frame in the low-power mode, the first station maycontrol the transmit power, based on the path loss obtained by addingthe maximum value and the path loss between the first station and theAP. Furthermore, when the transmitted frame is a data frame or responseframe in the low-power mode, the first station may control the transmitpower, based on the path loss between the first station and the AP.

At the step S303, the first station may additionally control thetransmit power depending on a data rate of the first station. That is,the first station may control the transmit power by additionallyconsidering a variable transmit rate.

The transmit power control method in accordance with the embodiment ofthe present invention may further include receiving data rateinformation from the AP or a station having received the transmittedframe of the first station. At the step S305, the first station maytransmit a frame according to a smaller data rate between the data rateof the first station and the received data rate, based on the transmitpower of the step S303.

FIG. 4 is a flow chart showing a transmit power control method of afirst station in accordance with another embodiment of the presentinvention.

Referring to FIG. 4, the transmit power control method in accordancewith the embodiment of the present invention starts from step S401.

At the step S401, the first station acquires a first path loss betweenthe first station and at least one or more stations included in the WLANsystem and a second path loss between the first station and the AP. Morespecifically, the first station may acquire the first and second pathlosses by using a TPG included in a frame transmitted from at least oneor more stations included in the WLAN system or the AP and an RSSI forthe frame transmitted from at least one or more stations or the AP.

At step S403, the first station controls a transmit power by using thefirst and second path losses. At this time, the first station controlsthe transmit power by using a maximum value of the first and second pathlosses or the second path loss.

At step S405, the first station transmits a frame according to thecontrolled transmit power.

More specifically, when the first station operates in the high-speedmode, that is, the transmitted frame is a data frame or response frameat the step S403, the first station may control the transmit power,based on the maximum value of the first and second path losses.Furthermore, when the transmitted frame is an RTS frame in the low-powermode, the first station may control the transmit power, based on themaximum value of the first and second path losses. Furthermore, when thetransmitted frame is a data frame or response frame in the low-powermode, the first station may control the transmit power, based on thesecond path loss.

At the step S403, the first station may additionally control thetransmit power depending on the data rate of the first station. That is,the first station may control the transmit power by additionallyconsidering a variable transmit rate.

Furthermore, the transmit power control method in accordance with theembodiment of the present invention may further include receiving datarate information from the AP or a station having received thetransmitted frame of the first station. At the step S405, the firststation may transmit a frame according to a smaller data rate betweenthe data rate of the first station and the received data rate, based onthe transmit power of the step S403.

FIG. 5 is a flow chart showing a transmit power control method of an APin accordance with another embodiment of the present invention. In FIG.5, the transmit power control method of the AP in the WLAN systemincluding at least one or more stations will be taken as an example.

Referring to FIG. 5, the transmit power control method in accordancewith the embodiment of the present invention starts from a step S501.

At the step S501, the AP acquires path losses between the AP andstations. More specifically, the AP may acquire a path loss by a TPGincluded in a frame transmitted from a station and an RSSI for the frametransmitted from the station or acquire a path loss between the AP and astation by receiving the path loss from the station.

At step S503, the AP controls a transmit power according to a maximumvalue among the path losses.

At step S505, the AP transmits a frame according to the controlledtransmit power.

At the step S503, the AP may additionally control the transmit powerdepending on the data rate of the AP. That is, the AP may control thetransmit power by additionally considering a variable transmit rate.

Furthermore, the transmit power control method in accordance with theembodiment of the present invention may further include receiving datarate information from a station having received the frame transmittedfrom the AP. At the step S505, the AP may transmit a frame according toa smaller data rate between the data rate of the AP and the receiveddata rate, based on the transmit power of the step S503.

In the transmit power control method in accordance with the embodimentof the present invention, the AP may transmit a beacon frame at themaximum power. When the transmit power of the beacon frame is controlledin accordance with the embodiment of the present invention, a newstation which is not included in a BSS may have difficulties in findingthe beacon frame and accessing the BSS. Therefore, the transmit powercontrol method in accordance with the embodiment of the presentinvention may further include transmitting a beacon frame at the maximumpower, thereby solving the above-described problem. The beacon frame istransmitted at an interval of 100 ms or more. Therefore, the beaconframe has a smaller effect upon the power consumption efficiency thanother frames such as a data frame.

Meanwhile, according to the transmit power control method of the AP inaccordance with the embodiment of the present invention, a probabilityin which an overlapping BSS (OBSS) problem occurs may decrease. When thetransmit power is controlled in accordance with the embodiment of thepresent invention, the radio range becomes narrow in comparison with themaximum power mode. Accordingly, a probability in which an intersectionarea with another BSS occurs may decrease.

However, when a BSS includes a general AP and station together with theAP and the station in accordance with the embodiment of the presentinvention, a signal collision problem may be caused by the general APand station. In particular, such a signal collision problem does notoccur in the second example of FIG. 1, but may occur in the firstexample of FIG. 1.

In order to solve such a problem, the transmit power control method ofthe AP in accordance with the embodiment of the present invention mayfurther include controlling the transmit power to decrease, when an OBSSexists for the BSS including the AP and the station. That is, when anOBSS exists after the transmit power is controlled at the step S503, thetransmit power may be reduced in such a manner as to minimize the OBSSarea.

Alternatively, the transmit power control method of the AP in accordancewith the embodiment of the present invention may further includechanging a channel with the station, when an OBSS exists for the BSSincluding the AP and the station. The transmit power control method inaccordance with the embodiment of the present invention may change thechannel with the station, when the OBSS area exists even though thetransmit power has been reduced because of the existence of the OBSS.

FIG. 6 is a diagram explaining a process of solving a hidden nodeproblem in accordance with the embodiment of the present invention.

FIG. 6 shows a process of solving a hidden node problem in a WLAN systemincluding a first AP AP1 and first to sixth stations STA1 to STA6. InFIG. 6, a circle indicated by a solid line represents the radio range ofthe first station, and a circle indicated by a dotted line representsthe radio range of the first AP.

In accordance with the embodiment of the present invention, the radioranges of the first AP and the first station may be varied. The firststation transmits an RTS frame to the first AP, and the first APtransmits a CTS frame. The first station acquiring a transmissionopportunity transmits data to the first AP. The second station receivingthe CTS frame transmits a response frame to the first AP.

When the minimum radio range is used in FIG. 6, the first and secondstations are included in the radio range of the first AP. Therefore, thehidden node problem does not occur for the first and second stations.However, the third to sixth stations become hidden nodes with respect tothe first and second stations.

In this case, the transmit power control method in accordance with theembodiment of the present invention may solve the hidden node problem byusing path loss information on the third to sixth stations. For example,when the fifth station is detected, the first station may control thetransmit power by using the path loss information between the firststation and the fifth station, such that the radio range is expanded. Inaccordance with the embodiment of the present invention, the transmitpower control method may minimize the hidden node problem, and maximizethe power consumption efficiency while maintaining the performance.

In FIGS. 1 to 6, the present invention has been described from theprocess point of view. However, the respective steps composing thetransmit power control method in accordance with the embodiment of thepresent invention may be easily understood from the apparatus point ofview. Therefore, the steps included in the transmit power control methodin accordance with the embodiment of the present invention may beunderstood as components included in a transmit power control apparatusin accordance with the principle of the present invention.

The transmit power control apparatus in accordance with anotherembodiment of the present invention includes a reception unit and acontrol unit. The reception unit is configured to receive path lossinformation from an AP, the path loss information containing a maximumvalue among path losses between the AP and at least one or more stationsincluded in a WLAN system. The control unit is configured to control atransmit power by using the path loss information. The control unitcontrols the transmit power by using a path loss obtained by adding themaximum value and a path loss between the AP and a station including thetransmit power control apparatus or the path loss between the AP and thestation including the transmit power control apparatus. Here, thetransmit power control apparatus may be included in a station, and thestation may transmit a frame according to the controlled transmit power.

A transmit power control apparatus in accordance with another embodimentof the present invention includes an acquisition unit and a controlunit. The acquisition unit is configured to acquire a first path lossbetween at least one or more stations and a station including thetransmit power control apparatus and a second path loss between an APand the station including the transmit power control apparatus. Thecontrol unit is configured to control a transmit power by using thefirst and second path losses. More specifically, the control unitcontrols the transmit power by using a maximum value of the first andsecond path losses or the second path loss between the AP and thestation including the transmit power control apparatus. The transmitpower control apparatus may be included in a station, and the stationmay transmit a frame according to the controlled transmit power.

A transmit power control apparatus in a WLAN system including one ormore stations in accordance with another embodiment of the presentinvention includes an acquisition unit and a control unit. Theacquisition unit is configured to acquire path losses between thestations and an AP including the transmit power control apparatus. Thecontrol unit is configured to control a transmit power according to amaximum value among the path losses. The transmit power controlapparatus may be included in the AP, and the station may transmit aframe according to the controlled transmit power.

In accordance with the embodiment of the present invention, the transmitpower is controlled according to required traffic and power consumptionefficiency, which makes it possible to improve the power consumptionefficiency and throughput. Furthermore, the transmit power is controlledaccording to the type of frame, which is possible to improve the powerconsumption efficiency and throughput.

Furthermore, the transmit power is controlled together with the linkadaption scheme used in the WLAN standards. Therefore, it is possible totransmit a frame at a more stable transmit rate.

Furthermore, as the transmit power of the AP is controlled, it ispossible to minimize the OBSS problem and the hidden node problem.

The above-described methods can also be embodied as computer programs.Codes and code segments constituting the programs may be easilyconstrued by computer programmers skilled in the art to which theinvention pertains. Furthermore, the created programs may be stored incomputer-readable recording media or data storage media and may be readout and executed by the computers. Examples of the computer-readablerecording media include any computer-readable recoding media, e.g.,intangible media such as carrier waves, as well as tangible media suchas CD or DVD.

While the present invention has been described with respect to thespecific embodiments, it will be apparent to those skilled in the artthat various changes and modifications may be made without departingfrom the spirit and scope of the invention as defined in the followingclaims.

What is claimed is:
 1. A method for controlling a transmit power in awireless local area network, the method comprising: establishing, by anaccess point (AP), a basic service set with a first station and a secondstation; and receiving, by the AP, a first Request To Send (RTS) fromthe first station, wherein the first RTS includes information about amaximum allowed transmit power used for communication between the AP andthe first station.
 2. The method of claim 1, further comprisingreceiving, by the AP, a second RTS from the second station beforereceiving the first RTS.
 3. The method of claim 2, further comprising:measuring, by the AP, a path loss between the AP and the second station;and transmitting, by the AP, to the first station, a first powerparameter indicating the measured path loss.
 4. The method of claim 3,wherein the maximum allowed transmit power is determined by the firststation based on the first power parameter and a second power parameterthat is measured by detecting the second RTS.
 5. An access point (AP)for controlling a transmit power in a wireless local area network, theaccess point comprising: a transceiver; and a controller operativelycoupled with the transceiver and configured to: establish a basicservice set with a first station and a second station; and instruct thetransceiver to receive a first Request To Send (RTS) from the firststation, wherein the first RTS includes information about a maximumallowed transmit power used for communication between the AP and thefirst station.
 6. The AP of claim 5, wherein the controller isconfigured to instruct the transceiver to receive a second RTS from thesecond station before receiving the first RTS.
 7. The AP of claim 6,wherein the controller is configured to: measure a path loss between theAP and the second station; and instruct the transceiver to transmit, tothe first station, a first power parameter indicating the measured pathloss.
 8. The AP of claim 7, wherein the maximum allowed transmit poweris determined by the first station based on the first power parameterand a second power parameter that is measured by detecting the secondRTS.